1. Field of the Invention
The present invention relates to an electromagnetic wave shielding material which contains carbon as a conductive ingredient.
2. Description of the Prior Art
As a highly advanced intelligent network is developed, electromagnetic waves generated from various kinds of electronic appliances and the like employing a digital technology have been remarkably closed. Such electromagnetic waves act on a computer, an industrial robot and the like to cause noises resulting in erroneous operations thereof, and therefore have been noted to cause serious environmental problems. To eliminate the problems caused by the electromagnetic waves, it is required both not to generate unserviceable electromagnetic waves from electronic devices and to keep the devices away from an influence of the electromagnetic wave from outside. In either case, an electromagnetic wave shielding material can provide an effective countermeasure. For the electromagnetic wave shielding material, a conductive compound plastic becomes noteworthy since it can be readily formed into any configuration of a device housing. The conductive compound plastic for use in the electromagnetic wave shielding material is generally a mixture of a resin and a conductive filler including powder, flake or fiber of a metal such as a copper, an aluminum, a zinc, a stainless steel and the like. However, the shielding materials employing the conductive metal material are still found to be unsatisfactory in that they cannot sufficiently shield a low frequency electromagnetic wave below 400 or 500 MHz as well as a high frequency electromagnetic wave above 600 or 700 MHz. In consideration of that carbon powder such as a graphite can be used as the conductive filler, studies have been made to investigate another electromagnetic wave shielding material comprising carbon powder mixed with a resin such as a phenol resin, and have revealed that the electromagnetic wave shielding material comprising the carbon powder intermixed with the phenol resin by means of a kneader and the like has an improved shielding effect rather than that employing the metallic conductive filler. Nevertheless, such shielding material is found to be still insufficient in shielding the low frequency electromagnetic waves below 400 or 500 MHz as well as the high frequency waves above 600 or 700 MHz.